refactor: Change hardcoded PX values in SCSS to REM where appropriate
As per #24 (closed), there's a desire to have hardcoded PX values changed to REM values, where appropriate, before handoff.
Simply enough, using the "16px == 1rem" rubric, this commit performs that conversion.
The exception is for breakpoint definitions, as my experience is that device widths tend to be expressed in PX rather than REM, and using PX for that purpose will be much more helpful to someone coming in to tweak a breakpoint value.
All other values - widths, heights, paddings and margins, font sizes, borders, et al - have been converted to REMs, doublechecked via calculator, and visually compared with staging.
Closes #24 (closed).